The invention relates to a method of decorating the surface of hard materials, especially materials which can be fired for the purpose of decoration, by direct or indirect printing using a colour paste containing a pigment and a thermoplastic medium (thermoplastic colour paste), the colour paste being printed onto the material to be decorated or onto a transfer material by means of inkjet technology. The method is directed especially towards the decoration of surfaces of glass, porcelain, enamel and other ceramic surfaces.
The production of decorations by printing directly or indirectly on an object to be decorated, including paper, metals, plastics, glass, porcelain, enamel and other ceramic materials, using pigmented colour pastes is known per se. In the case of the decoration of materials which can be fired for the purpose of decoration, such as glass, porcelain and other fired, biscuit-fired or unfired ceramic articles, application of the decoration to the surface to be decorated is followed by firing at from 500 to 1400xc2x0 C., whereby the decoration is fixed and, optionally, a partially fired or unfired article is also glazed. Although inkjet technology, a digital printing process, has become widely accepted in conventional printers for the office sector as well as in graphic technology, it has not yet been possible to use inkjet technology successfully for the decoration of ceramic materials.
In the inkjet process, droplets of colour, or ink, under the control of a digital data store, are applied pointwise to the material to be printed. The drops can be produced continuously or discontinuously. An overview of inkjet technology is given by David H. Fishman in American Inkmaker/June 1997, pages 36-39. In conventional inkjet printing processes, the printing inks used are organic colour solutions having a very low viscosityxe2x80x94inkjet inks having a viscosity in the range of from 1 to 10 mPaxc2x7s are usual. Although inkjet inks may also contain pigments, especially micropigments, in small amounts, it is then difficult to maintain the required low viscosity. The use of inorganic pigments and especially inks having a high content of inorganic pigments leads to problems in the inkjet printer because the specifically heavy pigments settle out readily. It is also known to use colour pencils in wax form instead of low-viscosity colours (xe2x80x9csolid inkjetxe2x80x9d). Such colour pencils in wax form also contain as the colouring component organic colourings or pigments dissolved therein having a density which is close to that of the wax surrounding them, generally, therefore, organic pigments such as carbon black, settling out of the pigments and hence problems with the functioning of the inkjet printer thus largely being avoided.
From WO 97/42040 there are known transfers, the decorative layer of which is applied by means of an inkjet printer. Single-colour or multi-colour inkjet printers may be used therefor. In addition to the conventional aqueous colour systems for inkjet printers, that process also uses so-called xe2x80x9csolid inksxe2x80x9d, that is to say systems of colours or pigments dispersed in a wax-like medium. That document gives no indication of whether the pigments are organic or inorganic pigments. That document does not indicate that the transfers can be applied to ceramic articles and the decoration can subsequently be fixed by decorative firing. This, as well as the fact that, to the knowledge of the inventors of the present Application, commercially available solid inks contain substantially no inorganic pigments, are indications that the pigments according to WO 97/42040 are substantially organic pigments and not inorganic pigments.
Attempts at printing ceramics colours by the inkjet process and hence making the inkjet process available also for decorating ceramic articles, such as glass, enamel and porcelain, have hitherto always failed owing to the pronounced tendency of the specifically heavy and coarse colour powders to form a sediment. The specific gravity of overglaze and glass colours is from 3.5 to 6.0 kg/1, and the mean fineness of grain of those products is from 3 to 5xcexcm. Those products settle out from aqueous or alcoholic suspensions having the conventional inkjet processing viscosity within a few seconds to the extent of in some cases 50%. Such suspensions would rapidly lead to blocking of the print nozzles and of the entire inkjet printing apparatus. Liquid colour pastes having a substantially higher viscosity, for example 5000 mPaxc2x7s, such as are used in screen printing for decorating glass, tend to separate and must be stirred again after standing for 15 minutes.
The object of the present invention is to provide a method by which colour pastes having a high content of inorganic pigments and/or ceramics decorative colours can be printed by inkjet technology. A further object is to decorate materials which can be fired for the purpose of decoration, such as glass, enamel, porcelain and other ceramic materials, using colour pastes containing ceramics decorative colours and using an inkjet process, the decoration being applied by direct or indirect printing to the material to be decorated. A further object consists in printing by means of inkjet colour pastes having a content of over 50 wt. % inorganic materials, especially inorganic pigments and glass flux. These and other objects which will become apparent from the description are achieved by the method according to the invention.
A method of decorating hard materials, especially materials which can be fired for the purpose of decoration, has been found, which method comprises applying to a surface of the material to be decorated a decorative layer based on a colour paste containing a pigment and a thermoplastic medium (thermoplastic colour paste) by means of direct or indirect printing, the colour paste being printed by means of inkjet technology, which method is characterised in that the colour paste, which contains at least 30 wt. % inorganic solids from the group of the pigments, glass frits and other glass-forming components, is applied to the surface of the material to be decorated or to a transfer material by means of a heatable inkjet print head, the temperature of which is kept above the melting point of the colour paste. The dependent claims relate to preferred embodiments and to those methods which are directed towards the decoration of materials which can be fired for the purpose of decoration.
Surprisingly, molten thermoplastic colour pastes, such as are used for decorating container glass, and accordingly have a very high content of inorganic solids, namely glass colours, exhibit virtually no settling out. Once the heating in the inkjet print head and/or in the supply element of the inkjet printing device has been switched off, the thermoplastic colour paste solidifies immediately, so that no sedimentation occurs even in the case of a long standing time.
The thermoplastic medium generally has a melting point in the range of from 30 to 100xc2x0 C., preferably from 40 to 80xc2x0 C. and especially from 45 to 60xc2x0 C. It was not foreseeable that thermoplastic colour pastes having a content of over 30 wt. % inorganic solids could be printed by means of inkjet technology without problems. Thermoplastic colours such as are to be used for the decoration of ceramic articles preferably contain over 50 wt. % inorganic solids.
Underglaze colours advantageously contain colour powders, that is to say mixtures of one or more inorganic pigments and a glass flux, in an amount of approximately from 65 to 75 wt. %, enamel, stoneware and porcelain colours in an amount in the range of from 70 to 80 wt. % and glass colours in the range of from 80 to 90 wt. %.
The thermoplastic colour pastes (that term is kept even though a pasty to liquid state of the colour pastes is not achieved until they have been melted) to be used according to the invention can contain one or more inorganic pigments of any desired structure and any desired chemical composition. For ceramics purposes, pigments which are substantially stable under the firing conditions are used. In addition to the pigments, the thermoplastic colour pastes may also contain glass frits or a combination of substances which fuse together under the firing conditions to form a glass. The glass-forming components are those which are known from glazing compositions, especially transparent glazing compositions. For the decoration of ceramics, the thermoplastic colour paste usually contains, in addition to the thermoplastic medium, a ceramics decorative colour, the person skilled in the art understanding by that expression a finely powdered mixture of silicate-like glasses (fluxes) and temperature-stable pigments. The production of decorative colours generally comprises grinding a mixture of the pigment(s) and the component(s) of the glass flux together.
The thermoplastic medium of the colour paste can contain one or more monomeric, oligomeric or polymeric compounds. Especially advantageous monomeric or, optionally, oligomeric compounds, which generally constitute the main constituent of the thermoplastic medium, are paraffins, fatty alcohols, fatty acids, fatty acid esters and fatty acid amides. In general, those compounds contain at least one aliphatic radical having at least 10 carbon atoms, preferably from 12 to 30 carbon atoms. Special preference is given to fatty alcohols having from 12 to 18 carbon atoms. As an alternative or in addition to the above-mentioned classes of substances, the thermoplastic medium may contain one or more polymeric or copolymeric compounds from the group of the polyolefins, polyoxyalkylenes, polyamides, polyesters, acrylate and methacrylate resins, natural resins and cellulose derivatives. According to a further alternative, the thermoplastic medium contains a combination of one or more of the aliphatic compounds and one or more of the polymeric compounds. According to an especially preferred embodiment, the thermoplastic medium contains one or more compounds from the group of the fatty alcohols, paraffins, hardened fats or polyoxyethylenes having a molecular weight of at least 10,000 as the main component and one or more compounds from the group of modified colophonium resins, methacrylate resins and cellulose derivatives, such as methylated, ethylated or carboxymethylated celluloses, as the subsidiary component.
A protective layer of a glass-forming material can additionally be applied to the decoration which has been printed onto the surface to be decorated or onto a suitable transfer material by means of an inkjet printing process. The glass-forming layer can likewise be applied by means of the inkjet method, or by means of another conventional method of producing protective layers. A protective layer is advantageous especially in the production of ceramics decorations, when the actual decoration is to be protected from external influences.
As has already been stated, it is also possible to use the method according to the invention to produce a transfer material, carrying the decoration, for indirect printing. The decorative layer applied to a transfer material, which is generally provided with a separating layer, or a combination of a decorative layer and a protective layer as well as a conventional organic film layer applied thereover, can be transferred to the surface of the material to be decorated by means of transfer, usually wet-stripping, by means of heat release or a conventional transfer printing process, for example a pad transfer printing process.
For printing the thermoplastic colour paste there are suitable conventional inkjet printers having a device for heating the inkjet print head. Advantageously, the inkjet print head is connected to a supply element, the supply element also being heated. In order to rule out sedimentation problems, it is advantageous to connect the print head and the supply element in such a manner that the molten colour paste can be circulated between the print head and the supply element. Accordingly, a preferred device for the inkjet printing of thermoplastic colour pastes having a high content of inorganic solids comprises an inkjet print head, a colour container and a hose pump; these elements are connected with one another in a loop and are under excess pressure, for example from 1 to 5 bar, during operation. The print head usually comprises a cube-shaped cavity (inkjet cell). One or more bores are arranged on the underside of the print head to form the print nozzles. The print nozzles are closed and opened from the inside by means of rubber plungers which are moved electromechanically (electromechanical unit). The colour paste is pumped into the print head from the side from a storage container, and the colour paste flows back into the colour container through a further opening.
The FIGURE shows a cross-section through an inkjet cell 1, the valve plunger 5 of which is shown in the closed position. A plurality of inkjet cells connected together in the manner of a bundle form the actual print head of the inkjet printing device. The cell comprises a base body 2, a nozzle 3, arranged in the base region, having the nozzle opening 4, the valve plunger 5 and a piezoelectric unit 6 permitting the to and fro movement of the plunger. The chamber 7 of the inkjet cell communicates via a channel 9 with a space 8 for supplying one or more inkjet cells with colour paste. The side wall 10 of the base body 2 contains a connection piece 11 for supplying the colour paste and a connection piece 12 for removing the same. The connection piece 11 is connected to a pump (not shown), and the connection piece 12 is connected to a supply element (also not shown), usually a storage container. To complete the loop, the pump and the supply element are connected to one another via a line (not shown). For melting the thermoplastic colour paste in the inkjet cell, the base body 2 is connected to a heating element 13, so that the colour paste in the chamber 7 and in the space 8 is in the pasty to liquid state. Advantageously, the supply element also contains a heating device.
For printing, the object to be printed is advantageously placed on an X-Y table beneath the print head. A control unit controls the opening and closing of the nozzles as well as the movement of the table containing the object to be printed. The object to be printed is at a distance of approximately 1 cm from the nozzle openings of the inkjet print head, and the colour paste emerging in drop form is placed accurately and thereby solidifies. It is also possible to keep the object to be decorated stationary and to arrange the inkjet print head to be movable.
In so far as a material which can be fired for the purpose of decoration has been printed directly or indirectly, the printing, or the transfer of the print image from a carrier to the object to be decorated, is followed in a known manner by firing. In the decoration of glass objects, the firing temperature is usually from 500 to 650xc2x0 C. An overglazing decoration is usually fired at from 700 to 900xc2x0 C., and an inglaze decoration is usually fired at a temperature of over 1100xc2x0 C.